Tishchenko Reaction Over Solid Base Catalysts

Abstract

Catalytic behavior of solid bases for mixed Tishchenko reaction in which an equimolar mixture of two different aldehydes is allowed to react was investigated employing the combinations of benzaldehyde and pivalaldehyde, pivalaldehyde and cyclopropanecarbaldehyde, and cyclopropanecarbaldehyde and benzaldehyde. The reactions were performed at 353 K for 4 h in vacuo without solvent using 5 mmol of each aldehyde and 100 mg of solid base catalyst. For all the combinations, the catalytic activity of alkaline earth oxides increased in the order of BaO≪MgO<CaO<SrO, implying that strongly basic site and high surface area are indispensable for exhibiting high activity. Treatment of benzaldehyde and pivalaldehyde with other solid bases such as La₂O₃, ZrO₂, ZnO, γ-alumina, hydrotalcite, KF/alumina, and KOH/alumina produced either no amount or very small amounts of cross-esterification and self-esterification products. Quantum chemical calculations carried out at the PM3-MO level for the positive charges on the carbonyl carbon atoms of aldehydes and the structure parameters of the active species for the ester formations indicated that the selectivities to four Tishchenko dimers over MgO and CaO are determined primarily in the step of the nucleophilic addition of the active species (PhCH₂O^-, ^tBuCH₂O^-, and C₃H₅CH₂O^-) to the carbonyl carbon atoms of aldehydes. The reaction of the aldehyde having more positively charged and sterically less hindered carbonyl carbon atom with the active species having less hindered oxygen atom proceeds faster.

We also attempted the application of solid base catalysts to the challenging Tishchenko reaction of furfural, and excellent results were obtained with CaO and SrO. For instance, when furfural (10 mmol) was treated with SrO (100 mg) without solvent at 353 K for 6 h in vacuo, almost quantitative conversion to the corresponding ester was accomplished. Furthermore, application of SrO to the Tishchenko reaction of 3-furaldehyde was carried out successfully. The catalytic systems were also successfully applicable to the intramolecular Tishchenko reaction (lactonization) of o-phthalaldehyde to phthalide. For example, treatment of o-phthalaldehyde (1 mmol) with CaO (50 mg) in benzene (1 mL) solvent at 313 K under N₂ produced phthalide quantitatively in a short time of 15 min. We finally refer to the perspective of application of solid base catalysts to Tishchenko reaction.